This invention relates to a spark plug igniter for intermittently firing a spark-discharge device, such as a spark plug in an internal combustion engine of an automobile and, more particularly, to the spark plug igniter provided with an ignition coil having a secondary winding connected in series to an auxiliary DC source so as to increase the effective ignition energy for minimizing the rate of miss-firing.
It has been known that an electrical spark in an internal combusion engine is a composite spark formed by a capacity spark discharge and a subsequent inductance spark discharge. In the capacity spark discharge, a large current flows for an extremely short duration with the result that the electromagnetic energy stored in the ignition coil is instantaneously discharged at a spark gap. However, in the inductance spark discharge which takes place immediately after the capacity spark discharge, a small current flows for a relatively long period determined by the self- and/or mutual-inductance of the ignition coil. Accordingly, the capacity spark discharge is closely related to the miss-firing ratio, while the inductance spark discharge to the capability of ignition.
In the conventional spark plug igniter, since the above-mentioned two kinds of discharges are actuated only by a high voltage induced across the secondary winding of the ignition coil, independent control or emphasis of the individual discharge is impossible. Accordingly, a proposal has been made in which an auxiliary DC or AC source is incorporated in series to the secondary winding of the ignition coil in such a manner that the voltage for inductance discharge is raised to increase the ignition energy. As a result of this construction, the miss-firing of the igniter is decreased to some extent, and better combusion of fuel is realized to improve the specific fuel consumption and to reduce harmful gas exhaustion. A constant voltage source with a low internal impedance is usually used as the auxiliary current source. In the igniter provided with such an auxiliary current source, the duration of an inductance spark discharge is extended as the output voltage of the auxiliary current source is increased. However, the igniter has the following disadvantages:
(1) If the voltage of the auxiliary current source is maintained at a constant value, the spark intensifying effect depends upon the number of revolutions of the engine, namely, the effect is reduced as the number of revolutions increases. Therefore, a voltage determined to obtain the sufficient spark intensity in a range of high number of revolutions (i.e. a high speed operation) becomes too high in a range of lower number of revolutions (i.e. a low speed operation), and results in (a) unstable sparking, (b) insufficient spark extinction, and (c) continuous spark. From this point of view, the source voltage must be determined to allow the low speed operation. However, the voltage so determined will not provide the satisfactory spark intensifying effect in the higher speed operation.
(2) If the auxiliary current source is constructed to have a constant voltage, the spark intensifying effect depends on the size of a plug gap. Since the spark intensifying effect decreases with increasing plug gap under a given number of revolutions of the engine, if the voltage is established at such point that the sufficient spark intensifying effect is obtained for a large plug gap, this voltage becomes too high for a small plug gap and causes the unfavorable result stated in the above item (1). Moreover, the gaps of the conventional plugs do not always have the same size and are destined to increase as the plugs wear, so that the voltage should be determined for a plug having a small gap or a new one. Accordingly, it cannot be expected to obtain the satisfactory spark intensifying effect in a case where the plug gap is widened.